Never been to a game in Montana, but I think it's safe to say that Autzen stadium is much louder. I think that Autzen was even louder before the expansion, but even with the expansion it's regarded as one of the loudest stadiums in the country. Back in 2000 when college gameday came to Eugene, Chris Fowler and Lee Corso mentioned that Autzen was the loudest stadium in the country (louder than Neyland, the Swamp, the shoe, etc.) Don't have the quote on me but Michigan coach Lloyd Carr said Autzen stadium was the loudest stadium he'd ever been in. Here's a quote from the Michigan Daily, 9/03-
"Autzen's 59,000 strong make the Big House [Michigan] collectively sound like a pathetic wimper. It's louder than 'The Swamp' at Florida, 'The Shoe' in Columbus and 'Death Valley' at Louisiana State. Autzen Stadium is where great teams go to die."
Autzen hasn't constantly been as loud of a stadium as it used to be, but I think some of that is with the frustration of the fans, and the fact that there wasn't much to cheer for this year. However, the Michigan and Arizona games are proof that the fans can still make Autzen into a very loud stadium. And the noise isn't just in the student section, I sit in one of the donor sections and it is just as loud there, as it is anywhere else in the stadium.